1. Field of the Invention
The present invention relates to a cathode ray tube, which forms a single screen by joining a plurality of divided screens for image display and an apparatus and a method of controlling brightness of an image displayed on an image display device such as a cathode ray tube.
2. Description of the Related Art
In an image display device such as a television receiver or a monitor device for a computer, a cathode ray tube (CRT), for example, is widely used. A cathode ray tube forms a scan screen according to scanning of an electron beam by irradiating an electron beam toward a phosphor surface from an electron gun provided inside of the cathode ray tube (simply called inside of the tube below). A cathode ray tube comprising a single electron gun is common. However, in recent years, a cathode ray tube with multiple electron guns has been developed.
In this type of cathode ray tube, a plurality of divided screens are formed by a plurality of electron beams emitted from a plurality of electron guns, and image display is performed by forming a single screen by joining the plurality of divided screens. A technology relating to the cathode ray tube comprising the plurality of electron guns is disclosed in, for example, Japanese Examined Utility Model Publication No. Sho 39-25641, Japanese Examined Patent Publication No. Sho 42-4928 and Japanese Unexamined Patent Publication No. Sho 50-17167. The cathode ray tube comprising the plurality of electron guns has such advantages that the depth can be shortened while the screen is enlarged compared to a cathode ray tube with a single electron gun. In order to join the plurality of divided screens, one screen may be obtained simply by joining an end portion of each divided screen linearly, or one screen may be obtained by partially overlapping adjacent divided screens. In FIGS. 23A and 23B, one example of a method for forming a screen is shown where one screen is obtained by overlapping adjacent end portions of two divided screens SL, SR. In this example, the center part of the screen is an overlapped region OL of the two divided screens SL, SR.
In addition to the cathode ray tube, one for forming a single screen by joining a plurality of divided screens for image display also has been developed as a projection type image display apparatus, for example. The projection type image display apparatus enlarges and projects an image displayed in a cathode ray tube or the like on a screen through a projection optical system. A technology related to such a projection type image display apparatus is disclosed in Japanese Examined Patent Publication No. Sho. 54-23762 and Japanese Unexamined Patent Publication No. Hei 5-300452, for example.
In the above-mentioned cathode ray tube with multiple electron guns, it is preferable that the joint area of the divided screens is as inconspicuous as possible when displaying a single screen in which the plurality of divided screens are jointed. However, in the related art, the technique for making the joint area of the divided screens inconspicuous is insufficient. For example, if the brightness is not adjusted properly in the joint area, differences in brightness are caused between adjacent divided screens, which is so called xe2x80x9cbrightness inconsistencies.xe2x80x9d In the related art, the technique for improving the brightness inconsistencies is insufficient. The brightness inconsistencies become a big problem in the overlapped region OL between the adjacent divided screens when a single screen is obtained by overlapping the adjacent divided screens SL, SR partially, as the example shown in FIGS. 23A and 23B.
A method for improving the brightness inconsistencies as mentioned above is described in a literature called xe2x80x9cSID digest p351-354 23.4: xe2x80x98The Camel CRTxe2x80x99,xe2x80x9d for example. The technology described in the literature will be explained with reference to FIGS. 23A and 23B. In this technology, a method is proposed where a video signal corresponding to the overlapped region OL on the screen is multiplied by a predetermined coefficient for correction depending on a position of a pixel in a horizontal direction (the direction of overlapping the screen, X direction in FIG. 23B), that is, a signal level of an inputted signal is changed depending on the position in the direction of overlapping screens for outputting. In this method, the level of the inputted signal for each screen corresponding to the overlapped region OL is corrected to a sine function, for example, such that a values in which brightness levels of the inputted signals at the same pixel positions Pi,j (Pi,j1, Pi,j2) on each of SL, SR screens overlapped is equal to brightness of an original image at the same pixel position, for example. However, while this method enables to improve the brightness of a part of the brightness area, it is difficult to improve the brightness all over the brightness area, as described in detail below.
Problems in the method of the related art for improving the brightness inconsistencies will be explained in more detail below. Generally, a brightness Y of a screen in a cathode ray tube or the like is expressed in a equation (1) below where a level of an inputted signal is D and a characteristic value for indicating a so-called gamma characteristic, gamma value, is xcex3. C is generally called perveance, which is a coefficient determined by a structure of an electron gun, for example.
Y=Cxc3x97Dxcex3xe2x80x83xe2x80x83(1)
The brightness distribution will be considered here, where two divided screens SL, SR are partially overlapped to form one single screen as the example shown in FIGS. 23A and 23B. Each brightness, Yxe2x80x21 and Yxe2x80x22, of the two divided screens SL, SR in the overlapped region OL can be expressed equations (2) and (3), respectively, similarly to the equation (1) above, where gamma values of the two divided screens SL, SR are xcex31, xcex32, respectively. In these equations (2), (3), k1 and k2 are coefficients for correction, to be multiplied to an inputted signal D corresponding to an overlapped region OL of a screen, depending on a pixel position Pi,j. Each of C1 and C2 is a predetermined coefficient corresponding to the coefficient C in the equation (1) above.
xe2x80x83Yxe2x80x21=C1xc3x97(k1xc3x97D)xcex31xe2x80x83xe2x80x83(2)
Yxe2x80x22=C2xc3x97(k2xc3x97D)xcex32xe2x80x83xe2x80x83(3)
Next, if a level of an inputted signal keeps the same value in the whole area of the screen, the brightness should be constant in the whole area, where degrees of brightness of the two divided screens SL, SR in the non-overlapped region are Y1 and Y2, respectively. Here, a condition for not causing the brightness inconsistencies described above can be expressed in an equation (4) below. Yxe2x80x21+Yxe2x80x22 is a value in which the degrees of brightness of the two divided screens SL, SR in the overlapped region OL are combined. When the equation (4) is solved, a relationship equation (5) below is derived.
Y1=Y2=Yxe2x80x21+Yxe2x80x22xe2x80x83xe2x80x83(4)
k1xcex31+k2xcex32=1xe2x80x83xe2x80x83(5)
Here, in the relationship equation (5) above, when the gamma values xcex31, xcex32 are constant values, the coefficients k1 and k2 for correction can be determined uniquely irrespective of a level of an inputted signal. However, in practice, since the gamma values depend on a level of an inputted signal and a degree of brightness of the screen, as shown in FIG. 24, they are not constant values.
A characteristic graph shown in FIG. 24 indicates a relationship between a level of an inputted signal (horizontal axis) and a degree of brightness (cd/m2) (vertical axis) actually observed on the screen. The graph was obtained by locally connecting measured points (xc2x7in FIG. 24) indicating values of inputted signals and values of the brightness with straight lines. In FIG. 24, the values of the inputted signals and the values of the brightness are indicated in logarithm (log). A gamma value xcex3 corresponds to a gradient of the graph (straight lines). Thus, if the gradient of the graph is constant irrespective of the level of the inputted signal, the gamma value xcex3 also would be constant irrespective of the level of the inputted signal. However, in practice, the gradient of the graph differs depending of the level of the inputted signal, and it is understood that the gamma value xcex3 differs depending of the level of the inputted signal. Therefore, in order to satisfy the condition expressed in the equation (5), the plurality of coefficients k1 and k2 for correction depending on the level of the inputted signal are needed.
Especially, in the case of a moving picture, since the level of the inputted signal varies dynamically, it is desirable to perform brightness control such that the coefficient for correction is dynamically changed to an appropriate one depending on the level of the inputted signal even at the same pixel position. However, in the related art, it is controlled by using a fixed coefficient, irrespective of the level of the inputted signal, and the coefficient for correction is not changed dynamically depending on the level of the inputted signal for control. Therefore, conventionally, while it is possible to improve the brightness in one brightness region, the brightness in the other brightness region is not improved.
In the Japanese Unexamined Patent Publication No. Hei-5-300452, the invention is disclosed where a control is performed by preparing, in order to achieve smoothing of the brightness in the overlapped region, a plurality of smooth curves for brightness control, which correspond to the coefficient for correction described above and selecting a curve corresponding to a characteristic and the like of an image projection device among the plurality of smooth curves. The invention disclosed in this publication, an appropriate curve is selected from the plurality of smooth curves, and then, the information of the selected particular smooth curve is stored in a non-volatile memory device to smooth brightness based on the stored smooth curve. By the way, in order to control the brightness depending on a signal level, a means is needed for detecting a signal level. In the publication described above, such a means for detecting a signal level is not disclosed or proposed. In the invention described in the publication above, since only the selected particular smooth curve is stored in the non-volatile memory device, it is obviously impossible to adjust the brightness dynamically while the image display device is in use. In the invention disclosed in the publication, the brightness control is performed with the same smooth curve as far as a new smooth curve is stored in the non-volatile memory device again.
As described above, in the invention described in Japanese Unexamined Patent Publication No. Hei-5-300452, it is not possible to perform the brightness control depending on a signal level. The invention disclosed in the publication is a technology for optimizing the brightness adjustment performed mainly in manufacturing, and it is not suitable for performing the brightness control in real time while the device is in use. Also, the invention disclosed in the publication, a video signal is controlled in an analog fashion by using the smooth curve. However, in order to adjust the brightness precisely, it is desirable to perform the brightness control digitally by using a correction coefficient, which is independent for each unit pixel or each unit pixel array. Further, the invention disclosed in the publication is optimized for a projection type image display device, and it is not suitable for being applied to one for performing direct image display through scanning of an electron beam.
Further, since the gamma value xcex3 is affected by factors other than an inputted signal, it is desirable to determine a coefficient for the brightness correction in view of other different factors. For example, since the gamma value xcex3 differs depending on a color, a different coefficient for correction is needed for each color in color display. Also, in a cathode ray tube, since a characteristic of the gamma value xcex3 differs depending on the difference in characteristics and the like of the electron guns, it is desirable to determine the coefficient for correction in view of the difference in the characteristics of the electron guns.
Further, as described below, it is desirable to change the coefficient for the brightness correction depending on a position of a pixel in the vertical direction (the direction orthogonal to the direction where screens are overlapped, (Y direction in FIG. 23B), in addition to that in the horizontal direction (the direction where screens are overlapped). The reason for it will be explained with reference to FIGS. 23A and 23B. Here, in the overlapped region OL, the brightness of pixels will be considered which exist in horizontally different positions A(1A, 2A), and B(1B, 2B). The degrees of brightness Yxe2x80x21A, Yxe2x80x21B in the positions 1A, 1B, respectively, where signal processing has been performed by using correction coefficients k1A, k1B on an input signal D, is expressed by equations (6) and (7) below as in the equation (1), where gamma values in the positions 1A and 1B in the left side divided screen SL are xcex31A, xcex31B, respectively. C1A and C1B are predetermined coefficients corresponding to the coefficient C in the equation (1).
Yxe2x80x21A=C1Axc3x97(k1Axc3x97D)xcex31Axe2x80x83xe2x80x83(6)
Yxe2x80x21B=C1Bxc3x97(k1Bxc3x97D)xcex31Bxe2x80x83xe2x80x83(7)
On the other hand, where gamma values at the positions 2A and 2B in the right side divided screen SR are xcex32A, xcex32B, respectively, the degree of brightness Yxe2x80x22A, Yxe2x80x22B at the positions 2A, 2B, after signal processing by using correction coefficients k2A, k2B has been performed on an input signal D, is expressed by equations (8) and (9) below. C2A, C2B are predetermined coefficients corresponding to the coefficient C in the equation (1).
Yxe2x80x22A=C2Axc3x97(k2Axc3x97D)xcex32Axe2x80x83xe2x80x83(8)
Yxe2x80x22B=C2Bxc3x97(k2Bxc3x97D)xcex32Bxe2x80x83xe2x80x83(9)
Here, a condition for not causing the brightness inconsistencies is expressed by equations (10), (11) below, when the degrees of brightness are Y1A, Y2A Y1B, Y2B at positions 1A, 2A, 1B, 2B, respectively, when image display is performed by only a single electron gun. Yxe2x80x21A+Yxe2x80x22A, Yxe2x80x21B+Yxe2x80x22B are a combined of brightness of the two divided screens SL, SR at the pixel positions A, B. Further, by solving the equations (10) and (11), relationship equations (12) and (13) are derived as shown in below.
xe2x80x83Y1A=Y2A=Yxe2x80x21A+Yxe2x80x22Axe2x80x83xe2x80x83(10)
Y1B=Y2B=Yxe2x80x21B+Yxe2x80x22Bxe2x80x83xe2x80x83(11)
k1Axcex31A+k2Axcex32A=1xe2x80x83xe2x80x83(12)
k1Bxcex31B+k2Bxcex32B=1xe2x80x83xe2x80x83(13)
Here, in a cathode ray tube, a transparency ratio and emitting light efficiency of light are different depending on a position of a phosphor surface. Therefore, as the gamma value xcex3 differs depending on a position of a phosphor surface, an equation (14) below holds. Further, from the equations (12)-(14), an equation (15) holds. From the equation (15), it is understood that it is desirable not only to control the brightness depending on a pixel position in the horizontal direction as done in the conventional manner, but also to control the brightness depending on a position in the vertical direction.
xcex31Axe2x89xa0xcex32A, xcex31Bxe2x89xa0xcex32Bxe2x80x83xe2x80x83(14)
k1Axe2x89xa0k2A, k1Bxe2x89xa0k2Bxe2x80x83xe2x80x83(15)
The present invention is made in view of those problems, and it is an object of the present invention to provide a cathode ray tube and an apparatus and a method of controlling brightness, mainly enables to control brightness of a plurality of divided screens properly depending on a signal level of a video signal so that a jointed part is not conspicuous.
A cathode ray tube of the present invention performs color image display by forming a single screen by joining a plurality of divided screens by partially overlapping each other. The plurality of divided screens are formed by scanning of the plurality of electron beams. The cathode ray tube comprises a signal dividing means for dividing an inputted video signal into a plurality of video signals for the plurality of divided screens, a storage means for storing a plurality of correction coefficients for each color corresponding to a plurality of signal levels, a signal level detection means for detecting a signal level of an inputted video signal for each color, and a calculating means for calculating an appropriate correction coefficient to be used for modulation control of brightness among a plurality of correction coefficients stored in the correction coefficient storage means. The cathode ray tube further comprises a brightness modulation means for performing control depending on a signal level on each of the plurality of video signals for divided screens by using a correction coefficient for each color calculated through the calculating means such that the sum total of degrees of brightness at the same pixel position in the overlapped region on the screen, which is scanned based on the plurality of video signals for the divided screens, is equal to a degree of brightness at the same pixel position on an original image and a plurality of electron guns for emitting a plurality of electron beams, which scans the plurality of divided screens based on a video signal for the divided screens where modulation control has been performed by the brightness modulation means.
Further, an apparatus for controlling brightness of the present invention performs brightness control of an image displayed in an image display device, which forms a single screen by joining a plurality of divided screens by partially overlapping each other. The apparatus for brightness comprises a signal level detection means for detecting a signal level of an inputted video signal, a storage means for storing a plurality of correction coefficients corresponding to a plurality of signal levels, and a calculating means for calculating an appropriate correction coefficient to be used for modulation control of brightness among a plurality of correction coefficients stored in the correction coefficient storage means based on the signal level detected by the signal level detection means. The apparatus for brightness control further comprises a brightness modulation means for performing control depending on a signal level on each of the plurality of video signals for the divided screens by using a correction coefficient calculated through the calculating means such that the sum total of degrees of brightness at the same pixel position in the overlapped region on the screen, which is scanned based on the plurality of video signals for the divided screens, is equal to a degree of brightness at the same pixel position on an original image.
Further, a method for brightness control of the present invention comprises steps of detecting a signal level of an inputted video signal, storing a plurality of correction coefficients corresponding to a plurality of signal levels in a storage means, calculating an appropriate correction coefficient to be used for modulation control of brightness, among a plurality of correction coefficients stored in the storage means; and performing modulation control of brightness depending on a signal level on each of the plurality of video signals for the divided screens by using a correction coefficient calculated so that a sum total of degrees of brightness at the same pixel position in the overlapped region on the screen, which is scanned based on the plurality of video signals for the divided screens, is equal to a degree of brightness at the same pixel position on an original image.
In the cathode ray tube and the apparatus and the method for brightness control according to the present invention, a plurality of correction coefficients associated depending on a plurality of signal levels are stored in the storage means, and an appropriate correction coefficient to be used for modulation control of brightness, is calculated among a plurality of correction coefficients stored in the storage means based on a signal level. Next, modulation control of brightness depending on a signal level is performed on each of a plurality of video signals for a divided screen by using the calculated correction coefficient so that a sum total of degrees of brightness at the same pixel position in the overlapped region on the screen scanned based on a plurality of video signals is equal to a degree of brightness at the same pixel position on an original image. As a specific example of the modulation control of brightness, operation processing is performed where a video signal is multiplied by a correction coefficient in order to change a degree of brightness.